Plate Heat Exchanger With Exchanging Structure Forming Several Channels in a Passage

ABSTRACT

The invention concerns a heat exchanger with brazed plates comprising a stack of parallel plates defining a plurality of generally flat fluid circulating passages, closure bars which delimit said passages and distributing means for distributing a fluid to each passage of a first series of passages and means for conveying another fluid to a second series of passages wherein at least one passage contains organized exchanging structures ( 15 ) which form a plurality of channels ( 19 ) in the width of the passage and also at least three channels ( 19 ) in the height of the passage. The invention is useful for air separation by cryogenic distillation.

The present invention relates to a plate and fin heat exchanger.

There are various types of plate and fin heat exchanger, each suited toa particular field of use. In particular, the invention appliesadvantageously to a heat exchanger of a unit for separating air or H₂/CO(hydrogen/carbon) mixtures using cryogenic distillation.

This exchanger may be a main exchange line of an air separationapparatus which cools the incoming air by indirect exchange of heat withthe cold products originating from the distillation column, asupercooler or a vaporizer/condenser.

The technology often used in these exchangers is that of brazed plateand fin aluminum exchangers, making it possible to obtain very compactcomponents with a large heat-exchange area.

These exchangers are made up of plates between which corrugated sheetsor fins are inserted, thus forming a stack of passages known as “cold”passages and passages known as “hot” passages.

The heat-exchange fins commonly used are straight fins, perforated finsand serrated fins.

These corrugated fins are characterized using the following parameters:

-   h(mm): height of the corrugated fin (from 3 to 10 mm)-   e(mm): thickness of the corrugated fin (from 0.2 to 0.6 mm)-   n (m⁻¹ or inch⁻¹) number of corrugated fins per unit length (from    177 to 1102 corrugations/m)-   perf(%): level of perforation (5% for perforated fins)-   l_(s)(mm): serration length (in the case of serrated fins)

Thus, the hydraulic diameters (Dh) of the fins conventionally used inbrazed plate and fin heat exchangers range between 1 and 6 mm. Thesecorrugated heat-exchange fins are currently formed using a press.

There are various ways of increasing the heat-exchange area.

The heat-exchange area which separates two fluids is made up of an areaknown as the “primary area” which corresponds to the flat area betweenthe two fluids and of an area known as the “secondary area” whichgenerally consists of fins perpendicular to the primary area and thusforming a corrugated heat-exchange fin. It is the number of finsinserted (the fin density) and the height of the fins which increase theheat-exchange area.

The denser the set of fins, the larger the heat-exchange area. However,there is a manufacturing limit and there are constraints associated withthe method. The press tool used to manufacture the corrugated set offins is able to obtain the maximum densities of 1023 to 1102corrugations per meter. The selected fin density may be lower when it ispreferable to limit pressure drops. In additions under certain operationconditions such as in bath-type vaporizer/condensers, constraintsassociated with safety limit the number of corrugations per meter tovalues well below the maximum values that can be achieved in themanufacture.

The fins have a temperature gradient. Beyond a certain fin height, theregion in the middle of the fin does not exchange heat anywhere near aswell. There is therefore an optimum fin height corresponding to anoptimum fin coefficient. The fin heights commonly used vary from 3 to 10mm.

It is also possible to increase the heat-exchange coefficient.

The more turbulent the fluid, the better the heat-exchange coefficient.This turbulence can be generated by altering the shape of the channelsor by inserting turbulence-generating obstacles (e.g. perforatedstraight fins, serrated fins, herringbone fins, louvered fins, or byinserting mini fins, apertures, etc.).

When a fluid is being vaporized, a surface which has a higher number ofnucleation sites exhibits a better heat-exchange coefficient. Thesenucleation sites are micro-cavities of various sizes and shapes(re-entrant cavities) present at the surface or through a porous layer.

When a fluid is being condensed, the thickness of the liquid film has anadverse effect on the heat-exchange coefficient. It is thereforeadvantageous to drain the liquid away using grooves, perforations orreliefs.

A type of heat exchanger known as a micro-scale heat exchanger hasrecently appeared.

This is an exchanger which has channels with hydraulic diameters smallerthan one millimeter. Reducing the size of the channels makes it possibleto expand the heat-exchange area (making the apparatus more compact).

The heat-exchange coefficient then becomes practically inverselyproportional to the hydraulic diameter.

S. Kandlikar in “First International Conference on Microchannels andMinichannels 2003, <Extending the applicability of the flow boilingcorrelation to low Reynolds number flows in microchannels>>” proposesthe following classification, based on the hydraulic diameter of thechannels:

-   mini-channels such that: 1 mm<Dh<3 mm (corresponding to the Dh    values of current corrugated fin sets)-   mini-channels such that: 200 μm<Dh<1 mm-   micro-channels such that: Dh<200 μm

For mini-channels (200 μm<Dh<3 mm): the laws of fluid dynamics forconventional pipes still apply.

For micro-channels (Dh<200 μm): the surface effects take on aconsiderable importance and the conventional laws of fluid dynamics nolonger apply.

EP-A-1008826 describes a plate-type heat exchanger in which at least oneof the passages contains tube-shaped closed auxiliary passages themaximum width of which is greater than 50% of the distance between twoadjacent plates.

The amount of flux exchanged across an exchanger is given by thefollowing equation:

φ=k×S×ΔT

For a given ΔT, exchanges can be improved only by increasing theheat-exchange coefficient (k) and/or by increasing the heat-exchangearea (S).

In the case of brazed plate and fin heat exchangers, increasing theheat-exchange area using a so-called “secondary” area reaches its limitsbecause of manufacture and/or constraints involved in the method.Increasing the heat-exchange coefficient by creating turbulence isadvantageous, but has two main pitfalls:

-   increasing the turbulence increases the pressure drops;-   it increases the cost of manufacture because of the complexity of    the geometry involved.

Thus, creating a new shape of corrugated fin set cannot increase theheat-exchange coefficient markedly beyond the levels achieved inexisting fin sets. As to creating nucleation and liquid drainage sites,these two methods relate only to a particular type of heat-exchange,mainly vaporization or condensation.

It would therefore appear to be difficult to make a substantialimprovement to brazed plate and fin heat exchangers by pursuingdevelopment along the same lines as described hereinabove.

Furthermore, technology of the micro-channel type is very expensive(micro-machining of the channels) and is currently reserved for verysmall-size heat exchangers: it does not at the present time apply toapplications such as the separation of air in which the throughput andthe temperature difference are high.

The proposed solution aims to increase the heat-exchange area byincorporating a third heat-exchange area known as the “tertiary area”into the already existing (“primary” and “secondary”) areas.

We are proposing three devices which make it possible to add a“tertiary” area to the corrugated heat-exchange fin sets currently usedin brazed plate and fin heat exchangers:

-   a “multiple corrugated fin set” exchange passage;-   “mini-channel” heat-exchange fin sets, extruded fin sets;-   “mini-channel” heat-exchange fin sets, capillary tubes.

One subject of the invention relates to a brazed-plate heat exchanger,of the type comprising a stack of parallel plates which define aplurality of fluid-circulation passages of flat overall shape, closurebars which delimit these passages and distributing means fordistributing a fluid to each passage of a first series of passages andmeans for sending another fluid to a second series of passages, in whichexchanger at least one passage contains at least one organized exchangestructure which forms a plurality of channels in the width of thepassager each channel being in contact with either at least two otherchannels or at least one other channel and one plate, the exchangerbeing characterized in that the structure also forms at least threechannels, and preferably at least five channels, in the height of thepassage.

As a preference, each channel is in contact with at least three otherchannels or one plate and two other channels. The plate may be a platedefining a passage or a secondary plate located in the passage.

According to other optional aspects:

-   the structure is made up of a plurality of cylinders;-   inside a passage, there is at least a secondary plate of flat    overall shape parallel to the plates that define the passages;-   the structure is formed of a superposition of corrugated    heat-exchange fin sets, each pair of adjacent corrugated    heat-exchange fin sets possibly being separated by a secondary    plate;-   the structure is formed of a single body containing a plurality of    channels;-   a channel has a hydraulic diameter of between 1 and 6 mm;-   a channel has a hydraulic diameter of between 200 μm and 1 mm;-   a channel has a hydraulic diameter of less than 200 μm;-   a passage has a height of between 3 and 18 mm;-   the channels have a circular, oval, square, rectangular, triangular    or diamond-shaped cross section.

Another subject of the invention is a cryogenic separation apparatuscomprising at least one exchanger as defined hereinabove.

Another subject of the invention is an air separation apparatus in whicha main heat-exchange line and/or a vaporizer-condenser and/or asupercooler is a heat-exchanger as described hereinabove.

The invention will be described in greater detail with reference to thedrawings in which:

FIG. 2 of the attached drawings depicts, in perspective with partialcutaway, one example of such a heat exchanger, of conventionalstructure, to which the invention applies.

FIGS. 3A, 4A and 5A depict a heat-exchanger passage viewed in thedirection of flow of the fluids according to the prior art and FIGS. 3B,4B, 4C and 5B depict a heat-exchanger passage viewed in the direction offlow of fluids according to the invention.

In FIG. 2, the heat exchanger 1 depicted consists of a stack of parallelrectangular plates 2, all identical, which between them define aplurality of passages for the fluids to be placed in an indirectheat-exchange relationship. In the example depicted, these passages are,in succession and cyclically, passages 3 for a first fluid, 4 for asecond fluid and 5 for a third fluid. It will be understood that theinvention covers heat exchangers involving just two fluids or heatexchangers involving any number of fluids.

Each passage 3 to 5 is flanked by closure bars 6 which delimit it,leaving inlet/outlet apertures 7 open for the corresponding fluid. Ineach passage, there are spacer corrugations or corrugated fins 8 whichact as thermal fins, as spacers between the plates, particularly at thetime of brazing, and as a way to prevent any deformation of the plateswhen using fluids under pressure, and as guides to guide the flow of thefluids.

The stack of plates, closure bars and spacer corrugations is generallymade of aluminum or aluminum alloy and is assembled in a singleoperation by furnace brazing.

Fluid inlet/outlet boxes 9 of semicylindrical overall shape are thenwelded to the exchanger body thus produced to fit over the correspondingrows of inlet/outlet apertures and are connected to pipes 10 supplyingand removing the fluids.

The channels can be formed using various techniques such as thosedescribed in “Micro échangeurs thermiques” by Anton GRUSS in “Techniquesde l'Ingénieur, 06-2002”.

The solution in FIG. 3B is to replace the conventional corrugatedheat-exchange fin set used in FIG. 3A with several corrugatedheat-exchange fin sets 13 of the same type, but with a shorter finheight. These new sets inserted in one and the same passage of the heatexchanger are assembled using thin sheets covered in braze metal 13.These sheets, termed “tertiary area sheets”, constitute the so-called“tertiary” added area. In the example, there are two sheets separatingthree fin sets.

All types of corrugated fin set that are commercially available can beused, merely by modifying and adapting the fin height. As a result, allthe parameters that make up the geometry of a type of corrugated fin setcan be adjusted (the thickness, density, perforation of the fin, etc.).The other parameters are:

-   the passage height,-   the number of exchanger fins per passage,-   the thickness of the tertiary area sheet (theoretically equal to the    thickness of the corrugated fin set),-   the shape of the tertiary area sheet: solid or with carefully    positioned perforations.

For this “multiple corrugated fin set” technology, the hydraulicdiameters are of the same order of magnitude as the width of the channelin a conventional corrugated fin (1/n−e)

The increases in heat-exchange area for various fin heights and bycomparison with a conventional fin set of equivalent density n are givenbelow:

Conventional configuration h h h passage e fin n w channel h fin channelincrease (mm) (mm) (m⁻¹) (mm) (mm) (mm) (mm) in area n* = 2 5.1 0.2551.18 1.61 4.9 2.45 2.25 19% 5.1 0.3 393.7 2.26 4.8 2.45 2.15 25% n* =3 7.13 0.2 944.88 0.86 6.93 2.24 2.04 12% 7.13 0.2 629.92 1.39 6.93 2.242.04 24% n* = 4 9.63 0.2 944.88 0.86 9.43 2.26 2.06 13% 9.63 0.2 629.921.39 9.43 2.26 2.06 27% n* = number of fins over the height of a passage(with tertiary area sheet thicknesses of 0.2 mm) w = channel width hchannel = channel height

We are restricted here to channel heights (h channel) of a minimum of 2mm (for brazing reasons).

For the same volume, increasing the number of fins stacked up in theheat exchanger increases the cost of manufacture thereof. However, theinstallation cost remains the same.

The solution in FIG. 4B is to replace the conventional corrugatedheat-exchange fin set used in FIG. 4A with a structured corrugated finset 17 comprising numerous mini-channels 19 of square cross section.This corrugated fin set can be produced by extrusion.

The extrusion manufacturing method means that it is possible to conceiveof any channel cross section (rectangular, triangular, round,diamond-shaped, etc.). FIG. 4C shows channels of triangular crosssection.

The main parameters are the height of the passage, the number ofchannels per passage height, the number of channels per meter width ofpassage and all the parameters involved in the geometric shape of thechannels used (channel height, width, diameter, etc.).

This method of manufacture also allows the possibility of insertingmicro-fins or mini-fins inside the channels in order further to increasethe heat-exchange area and/or to drain away a liquid.

The length of the channels (fluid exchange length) can be divided intoseveral extruded corrugated fin set modules spaced a few millimetersapart so as to allow inter-channel communication.

There are three categories of channel geometry differentiated in termsof the hydraulic diameter (Dh) of the channels:

-   -   channels such that Dh is of the same order of magnitude as the        width of the channels in conventional corrugated fin sets        (w=1/n−e);    -   channels such that Dh ranges between 200 microns and 1 mm        (mini-channels);    -   channels such that Dh is less than 200 microns (micro-channels).

The increases in heat-exchange area obtained for the three categoriesmentioned hereinabove are as follows:

For channels such that Dh is of the same order of magnitude as the widthof the channels in conventional corrugated fin sets (w=1/n−e), we arehere quoting the increases in heat-exchange area (se) for various finheights and with respect to a conventional fin set of the same heightand equivalent density n.

Extruded structure h n w se channel (m⁻¹) (mm) (m²/m²) (mm) increaseConventional corrugated fin set h = 5.1 mm 551.18 1.61 7.18 2.25 19%393.7 2.26 5.51 2.25 33% Conventional corrugated fin set h = 7.13 mm944.88 0.86 14.73 0.96 40% 629.92 1.39 10.48 1.53 40% Conventionalcorrugated fin set h = 9.63 mm 944.88 0.86 19.44 0.97 43% 629.92 1.3913.63 1.69 42%

For the channels such that Dh ranges between 200 microns and 1 mm(mini-channels), we are here quoting the increases in heat-exchange area(se) for various fin heights and with respect to a conventionalcorrugated fin set of the same height and with a high density n.

Extruded structure h n se channel (m⁻¹) (m²/m²) (mm) increaseConventional corrugated fin set h = 5.1 mm 1 102.36 12.36 0.2 161%Conventional corrugated fin set h = 7.13 mm 1 102.36 16.84 0.2 171%Conventional corrugated fin set h = 9.63 mm 1 123.62 20.86 0.2 197%

For the channels such that Dh is less than 200 microns (micro-channels),we are here quoting the increases in heat-exchange area (se) for variousfin heights and with respect to a conventional corrugated fin set of thesame height and with a high density n.

Extruded structure h n se channel (m⁻¹) (m²/m²) (mm) increaseConventional corrugated fin set h = 5.1 mm 1 102.36 12.36 0.05 717%Conventional corrugated fin set h = 7.13 mm 1 102.36 16.84 0.05 741%Conventional corrugated fin set h = 9.63 mm 1 123.62 20.86 0.05 818%

The solution in FIG. 5B is to replace the conventional corrugatedheat-exchange fin set used in FIG. 5A with a suitable number ofcapillary tubes. The capillary tubes can be easily arranged in anordered fashion because of their shape. The capillary tubes are coveredwith braze metal to mechanically assemble the whole.

The adjustable parameters are the height of the passage, the diameter ofthe capillary tubes, the thickness of the capillary tubes or the numberof capillary tubes per m²

We are here quoting the increases in heat-exchange area (se) for variousfin heights and with respect to a conventional corrugated fin set ofequivalent density. D_(ext) is the external diameter of the capillarytube.

Solution: 7 capillary tubes per Conventional corrugated fin set passageheight h passage (mm) n (m⁻¹) D_(ext) (mm) increase in se 5.1 551.18 1.450% 5.1 393.7 1.4 96% 7.13 944.88 1.2 23% 7.13 629.72 1.2 73% 9.63944.88 1.4 10% 9.63 629.72 1.4 57%

In each example, the diameter of the capillary tube corresponds to themaximum diameter in order to obtain an increase in heat-exchange areawith respect to the conventional solution; a smaller diameter will givea far less pronounced increase in heat-exchange area.

1-11. (canceled)
 12. A brazed-plate heat exchanger, of the typecomprising a stack of parallel plates which define a plurality offluid-circulation passages of flat overall shape, closure bars whichdelimit these passages and distributing means for distributing a fluidto each passage of a first series of passages and means for sendinganother fluid to a second series of passages, in which exchanger atleast one passage contains at least one organized exchange structurewhich forms a plurality of channels in the width of the passage, eachchannel being in contact with either at least two other channels or atleast one other channel and one plate, the exchanger being wherein thestructure also forms at least three channels in the height of thepassage.
 13. The exchanger of claim 12, in which the structure is madeup of a plurality of cylinders.
 14. The exchanger of claim 12,comprising, inside a passage, at least a secondary plate of flat overallshape parallel to the plates that define the passages.
 15. The exchangerof claim 12, in which the structure is formed of a superposition ofcorrugated heat-exchange fin sets, each pair of adjacent corrugatedheat-exchange fin sets possibly being separated by a secondary plate.16. The exchanger of claim 12, in which the structure is formed of asingle body containing a plurality of channels.
 17. The exchanger ofclaim 12, in which a channel has a hydraulic diameter of between 1 and 6mm.
 18. The exchanger of claim 12, in which a channel has a hydraulicdiameter of between 200 μm and 1 mm.
 19. The exchanger of claim 12, inwhich a channel has a hydraulic diameter of less than 200 μm.
 20. Theexchanger of claim 12, in which the channels have a circular, oval,square, rectangular, triangular, or diamond-shaped cross section.
 21. Acryogenic separation apparatus comprising at least one exchanger of asclaimed in claim
 12. 22. The air separation apparatus of claim 21, inwhich a main heat-exchange line and/or a vaporizer-condenser and/or asupercooler is said brazed-plate heat exchanger, of the type comprisinga stack of parallel plates which define a plurality of fluid-circulationpassages of flat overall shape, closure bars which delimit thesepassages and distributing means for distributing a fluid to each passageof a first series of passages and means for sending another fluid to asecond series of passages, in which exchanger at least one passagecontains at least one organized exchange structure which forms aplurality of channels in the width of the passage, each channel being incontact with either at least two other channels or at least one otherchannel and one plate, the exchanger being wherein the structure alsoforms at least three channels in the height of the passage.